Impelled by environmental concerns and increasing governmental regulations on the volatile organic content of coatings, the applications of water-based flexographic and rotogravure inks are increasing in the ink industry. A typical water-based ink system is formulated by the addition of a binder resin (usually acrylic polymer latices) to a grind base. Grind bases are prepared by using grinding resins to disperse pigments. Pigments are crystalline solids composed of agglomerates, aggregates, and primary particles which vary in size from 0.02 to 0.50 microns. During the grinding process, agglomerates and aggregates are broken down into primary particles which possess strong tendencies to reassociate in an ink. The finer the particle size of the pigment, the greater the color strength, but the more difficult the pigment becomes to disperse. Grinding resins help prevent the particles from reassociating by increasing both the electrostatic and steric repulsion between pigment particles.
An ink formulator must consider the compatibility of the various ink components when selecting the grind resins to be used. Grind bases (i.e., pigment dispersions) are let down with a variety of alkali-soluble resins or alkali-insoluble resin emulsions to achieve the properties desired for the end use of the ink. If the grind resins and the let-down resins are not compatible, the result may be pigment flocculation, viscosity increase, loss of color strength, and other problems.
Soluble maleic resins have been used for several years in water-based inks and coatings. Typically these resins are partial esters of maleated or fumarated rosin with various polyols having acid numbers greater than 140. However, when used in aqueous pigment grinding media, these traditional maleic resins exhibit a major shortcoming--the viscosities of the resultant pigment dispersions tend to be unstable over time.
It is known to correct this problem by modifying maleic resins with styrene-allyl alcohol (SAA) copolymer, thereby producing pigment grinds with stable viscosities. SAA is a hard (softening point of 95.degree.-110.degree. C.) thermoplastic, low molecular weight polymer manufactured by the Monsanto Corporation. However, SAA copolymers have two significant disadvantages: 1) they are relatively expensive, and 2) they are currently commercially available in only two grades--RJ-100 (which has a molecular weight of 2340) and RJ-101 (which has a molecular weight of 1700). Thus, the range of maleic resins which may be formulated utilizing SAA is severely limited.
Therefore, it is the object of this invention to formulate water-based inks and coatings containing stable viscosity pigment grinds produced from soluble maleic grinding resins which are not SAA modified.